1. Field of the Invention
This invention relates generally to an optical proximity switch.
2. Description of the Prior Art
Optical proximity switches are used as contactless electronic switchgear for surveillance purposes and in automation technology. They include an optical transmitter, usually a light emitting diode (LED) or laser diode, for emitting a light signal into a surveillance area, and at least one optical receiver for receiving this light signal. If an object is located in the surveillance area, the light signal is interrupted or reflected, or at least attenuated or varied in its polarization direction.
If the variation of the light signal by the object reaches an extent defined by a switching threshold, a switching signal is tripped, i.e., causing some action to take place. In addition to such binary switchgear, optical sensors output the measured variable, in particular the reflectance and/or the location of the object as an analog signal.
Switchgear of this kind is produced and sold in the most various versions, including by the present Applicant.
To define the surveillance area more precisely, but also to improve the sensitivity, both the transmitter and the receiver are provided with preferably a collimating lens. In addition to the beam formation, a critical aspect is to make optimal use of the volume available in the device.
In the past, refractive lenses of glass or plastic were preferentially used, provided with reflection-reducing coatings as needed. To save space and weight, Fresnel lenses were used. For wide-open optical s with a narrow field angle, parabolic mirrors were also used. Especially in the transmitting lenses, in contrast to the imaging lenses, crucial factors were uniform lighting, a defined field boundary, and good utilization of the beam power available. A certain lack of sharpness is even desirable here, for the sake of blurring the structure of the light-emitting surface, in particular the bonding pad usually located in the center of the chip.
U.S. patent application 2010/0085580 A1 (DE 10 2006 055 743 B4) shows an optical sensor with a lighting, comprising a plurality of elements, which has a free-form surface for varying the distribution of brightness in the transmitter spot. Besides the comparatively high cost for material, the unsatisfactory efficiency of the transmitting lens must be considered disadvantageous. Since the LED chips preferably have a Lambertian radiation pattern, only some of the radiation capacity can be captured, given the opening angles attainable with an imaging lens. At an opening ratio of 1:1.4 (opening angle 2w=40°), only approximately 66% of the radiation capacity is utilized. Approximately one-third is lost.
U.S. Pat. Nos. 5,577,492 A1 and 7,874,703 B2 disclose collimating lenses suitable for lighting purposes that have total internal reflection surfaces for deflecting the outer focused beams in the direction of the optical axis (total internal reflection lenses). Thus even light beams from the range above 40° can be used for lighting. However, then the optical imaging is largely lost, making them appear unsuitable for optical proximity switches. This is especially true whenever the geometry of the light distribution on the receiver surface is evaluated, as in proximity switches that operate on the principle of triangulation, and in one-way light gates with evaluation of the beam geometry.
U.S. Pat. No. 7,942,553 B2 shows a typical application of the aforementioned lens for lighting purposes.